Tune KeywordList lookup & avoid allocations for implicit capture group

Summary:
No longer any allocations, we use sorted vectors for lookup.
Init sorted vectors lazy, but no longer as lazy that mutable members are needed.

Test Plan:
make && make test works
perf looks nicer afterwards

Reviewers: vkrause, dhaumann

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D15217